Glass ferrule optical fiber connectors

ABSTRACT

The specification describes processes for the manufacture of glass ferrules for optical fiber connectors wherein the glass ferrules have a composition comprising silicon oxide, alkali metal oxide, and aluminum oxide to which is added 1-35% of lead oxide.

RELATED APPLICATION

This application is a division of application Ser. No. 09/089,155 filedJun. 2, 1998, now U.S. Pat. No. 6,151,916 issued Nov. 28, 2000.

FIELD OF THE INVENTION

This invention relates to glass ferrules for connecting optical fibers,and to methods for their manufacture.

BACKGROUND OF THE INVENTION

Optical fiber connectors that comprise a glass ferrule are known. SeeU.S. Pat. No. 4,850,670. However, despite potential cost advantage overconventionally used ceramic ferrules, glass ferrules have found onlylimited use, e.g., in the so-called rotary splice. This general failureto adopt an otherwise advantageous technology is due at least in part bythe failure of many prior art glass ferrules to meet stringentmechanical requirements, including strength and dimensional standards.Indeed, in the rotary splice there is only minimal mechanical stress onthe glass ferrule since the rotary splice is designed for one timeassembly.

Glass ferrules are produced typically from a tubular preform by drawingthe preform into a continuous glass tube, and cutting the tube intosections each of which becomes a glass ferrule. Since the earlyrecognition of the potential economies of substituting glass ferrulesfor ceramic ferrules, one concern about reliability of glass ferrulemanufacture has been the dimensional control capabilities of glassmaking technology as compared with the known dimensional precisioninherent in ceramic technology. In practice, it has been found thatrelatively good dimensional control can be realized with glass ferrulefabrication techniques. This is due to inherent behavior of glass duringtube drawing in which the geometry of the preform is replicated to ahigh degree in the drawn tube, and the success of glass ferruletechnology so far has relied on that inherent property. However, anotherconcern with glass ferrules is strength. Considerable efforts have beenmade to improve the strength of glass materials for ferrule manufacture.

In view of the significant cost savings that can be realized from thereplacement of ceramic ferrule optical fiber connectors with relativelyinexpensive glass ferrule optical fiber connectors, it would be highlydesirable to have available glass ferrules with improved strength thatcan meet the design standards for current connectors, and also have thedimensional control necessary to meet those standards.

A technique for producing high strength glass ferrules for optical fiberconnectors is described and claimed in U.S. Pat. No. 5,598,496. Thistechnique involves etching the outer surface of the glass ferrule toimprove the strength of the glass, and coating the etched surface withan adherent coating of, e.g. Ni and Au.

U.S. Pat. No. 5,295,213 discloses a method of strengtheningalkali-containing glass ferrules by ion exchange. The ion exchangemethod applies to borosilicate glass containing substantial amounts ofNa₂O, and results in a thin layer of strengthened glass on the outersurface of the glass where the ion exchange process occurs. However,this layer is thin, and is often abraded away in practical service afterwhich the ferrule returns to its original weak state. Moreover, thistechnique is not applicable to vitreous silica or PYREX™ ferrules.

It is known that glasses with a higher amount of sodium and with asignificant amount of alumina are more effective when treated by anion-exchange process, and we have used such glass compositions, e.g.those described in U.S. Pat. No. 3,661,545, to make ferrules that cansurvive very harsh abrasion treatment and thermal shock with onlymoderate loss of the enhanced strength.

Although the glass materials described in U.S. Pat. No. 3,661,545 arehighly desirable in terms of strength and overall utility, they aredifficult to process. They melt at very high temperatures, i.e.1500-1550° C., which are inconvenient from a manufacturing standpoint.Moreover, even when melted at this temperature, they retain many smallbubbles and typically have unmelted stones and cords (compositionalnon-uniformities). When ferrules are drawn from a preform with thesecharacteristics, the defects in the glass cause irregularities in thedrawing process, confuse control equipment, and drastically reduce theyield of ferrules within required dimensional tolerances. Moreover, thehigh melting characteristics of these glass lead to high extrusiontemperatures, i.e. nearly 1000° C., in preparing the ferrule preforms.This unusually high extrusion temperature rapidly deterioratesextrusions dies, thus resulting in higher costs of manufacture.

A process for improving the manufacturability of high strength glassferrules would be a significant advance in this technology.

STATEMENT OF THE INVENTION

I have developed a new glass material that can be strengthened usingconventional ion exchange processes, and melts at temperatures that areconvenient for economical manufacture. This glass material is a sodiumaluminum silicate glass modified with lead oxide.

BRIEF DESCRIPTION OF THE DRAWING

The Figure shows a typical optical fiber connector employing a glassferrule component.

DETAILED DESCRIPTION

A typical optical fiber connector with a glass ferrule connectorcomponent is shown in FIG. 1. The connector comprises glass ferrule 11with a center bore 14 for the optical fiber (not shown). It should beunderstood that the drawing is not to scale. For example, the bore inthe glass ferrule is exaggerated for clarity. The ferrule is adapted forinsertion into a terminal member 12 which has a center bore 13 for themating fiber. To accommodate a coated fiber the bore 13 in the terminalmember 12 is typically larger than the bore 14 in ferrule 11. Examplesof these types of connectors are described in U.S. Pat. Nos. 4,850,670;5,396,572; 5,295,213 and 4,812,009, all of which are incorporated hereinby reference.

As mentioned above, glass ferrule components such as 11 in FIG. 1, canbe manufactured from a hollow bore preform by drawing the preform into atube, as described in U.S. Pat. No. 5,295,213, and cutting sections ofthe drawn tube to form glass ferrules. As is well known the preform hassubstantially larger dimensions than the drawn tube and these dimensionsdetermine the geometry and the dimensional precision of the drawnferrules. Due to the requirement of a hollow bore, preforms for glassferrules are typically formed by extrusion using well known glassextrusion techniques, although machining and drilling from cast ingotsare alternative choices.

Glass materials for ferrule manufacture are preferably subjected to anion exchange process to strengthen the outer shell of the glass ferrule.After forming the glass ferrule body, i.e. after drawing from thepreform and preferably after cutting the individual ferrule lengths, theferrules are treated in a molten salt bath of, for example, potassiumnitrate to exchange potassium ions for sodium ions in the glass ferrulebody. This effect takes place in a surface layer of the glass body bydiffusion of alkali ions both into and from the glass body. Thisinterdiffusion results in an increase of large potassium ions in thesurface layer which causes the surface of the glass body to be stressedin compression which as known in the art imparts improved mechanicalstrength to the glass body. The depth of the potassium rich layerdepends in part on the duration of the ion exchange process. While veryhigh strength has been demonstrated even for thin (5-10μm) layers,slight abrasion or polishing may remove this layer and eliminate thestrengthening effect. Accordingly thicker layers, >20 μm, are preferred.

The glass material forming the ferrule structure according to theinvention is a sodium aluminum silicate glass to which substantialamounts of lead oxide are added. The addition of lead oxide in amountsof 1-35% is found to improve the manufacturability of glass ferruleswithout impairing their mechanical strength. Examples of these glassmaterials were prepared and processed as in commercial ferrulemanufacture. The examples are presented in the following Table I. Theamounts are in weight %.

TABLE I Components Example 1 Example 2 Example 3 SiO₂ 55.3 50.3 51.35Na₂O 11.4 10.4 10.6 K₂O 3.25 2.95 3.05 MgO 3.25 2.95 — CaO 0.2 0.2 —Al₂O₃ 15.15 13.8 14.0 PbO 9.9 18.0 20.1 TiO₂ 0.65 0.6 — As₂O₃ 0.9 0.80.9

The softening points, anneal points, and strain points for these glasseswere measured and the results are given in Table II.

TABLE II Properties Example 1 Example 2 Example 3 Softening pt, ° C. 824775 788 Annealing pt., ° C. 580 563 546 Strain pt., ° C. 530 519 498

Measurements of flexural strength were performed on ferrules after anion exchange process. Ion exchange processes are well known in the art,and typical conditions are heating to a temperature of at least 350° C.and a treatment period of at least 15 minutes. The same ferrules werethen subjected to a severe attrition process in a hard SiC powder. Thisattrition process causes microcracks on the glass surface and usuallysubstantially degrades the strength of the glass. The original glass,without the ion exchange strengthening process, shows strengthdegradation from about 40 Kpsi to about 18 Kpsi after a 20 min.attrition process. For comparison, flexural strength measurements of theion exchanged glasses were taken after a 20 minute and a 2 hour exposureto this treatment. The results are given in Table III below. Theconditions of the ion exchange process are indicated in the Table.

TABLE III Properties Example 1 Example 2 Example 3 Ion Exch. @ ° C./hr450/1  425/16 425/16 Flex. strength psi 209k ± 12k 191k ± 23k 191k ± 30kno attrition Same after 20 min. 168k ± 33k 158k ± 8k  164k ± 19kattrition in SiC Same after 2 hr. 171k ± 31k attrition in SiC

As seen from Table III, all ion-exchanged specimens exhibit somedegradation of strength after attrition (15-20%). Still the residualstrength remains very high.

The processing temperatures implicit from the measurements in Table IIcompare favorably with the high strength glass compositions described inU.S. Pat. No. 3,661,545. Compared with the properties of those materialsthe glasses of this invention have a softening point and annealing pointof the order of 50-100° C. lower while retaining at least as goodmechanical (i.e. strength) properties.

Based on the foregoing examples the relative amounts of silicon oxide,alkali metal oxide, lead oxide and aluminum oxide than can be predictedwith reasonable confidence to yield high strength glass ferrules withlow processing temperatures are:

alkali metal oxide (as A₂O): 9-17 wt % - preferably 11-15 wt % aluminumoxide (as Al₂O₃) 9-18 wt % - preferably 11-16 wt % lead oxide (as PbO)1-35 wt % - preferably 7-30 wt % silicon oxide (as SiO₂): perferably45-65 wt %

The alkali metal A (in A₂O) can be selected from the group consisting ofNa, Li and K but the A₂O ingredient should contain at least 9% Na₂O, andpreferably is a mixture of Na₂O and K₂O. Other oxides, such as CaO, MgO,TiO₂ may be found useful in small amounts. They typically will compriseless than 5% of the overall composition. As₂O₃ and Sb₂O₃ are frequentlyincluded in an amount of approximately 1% to reduce bubble formation.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

I claim:
 1. A glass ferrule comprising a cylindrical glass body having aconcentric cylindrical bore said cylindrical glass body having acomposition comprising silicon oxide, alkali metal oxide, lead oxide andaluminum oxide in the following weight %: alkali metal oxide (as A₂O):9-17 wt % aluminum oxide (as Al₂O₃) 9-18 wt % lead oxide (as PbO)20.1-35 wt % silicon oxide (as SiO₂): remainder where A is selected fromthe group consisting of Li, Na and K and A₂O comprises at least 9 wt %of Na₂O.
 2. The glass ferrule of claim 1 wherein the composition of thecylindrical glass body includes additional additives up to a total of 5wt % of the total composition.